1. Field of the Invention
The present invention relates generally to fluid flow sensors and, more particularly, to such a sensor for detecting very low flow rates.
2. Description of the Prior Art
In the field of flow rate sensing and monitoring, a well-known form of sensing device takes the form of a paddle wheel inserted into the stream of fluid flow with the rate of rotation of the paddle wheel being proportional to fluid flow and electronically sensed and processed to produce an indication of the rate of fluid flow. In some applications, the paddle wheel sensor is inserted into a relatively large pipe or conduit through which the fluid flows. For other applications utilizing relatively small conduits and tubing, the paddle wheel sensor is a separate element through which the fluid itself flows. In certain of such applications, particularly when very low fluid flow rates are to be sensed, special constructions are employed to insure that the paddle wheel rotates in accordance with the actual fluid flow, even though the fluid velocity is relatively small.
Two prior art constructions for the detection of relatively low fluid flow rates are illustrated by the patents to Wemyss U.S. Pat. No. 3,866,469 and Blise et al U.S. Pat. No. 3,981,194, in which the fluid inlets and outlets are on the same side of a fluid chamber containing the paddle wheel and the inlet and outlet are in relatively close proximity to each other. In such a construction, the fluid flow around the paddle wheel is essentially circumferential in that the fluid maintains a reactive contact with the paddle wheel for a substantial portion of the rotation of the wheel.
While such prior art constructions have utilized basic concepts needed for the measurement of very low fluid flow rates, such as substantially circumferential rather than tangential fluid reaction with the paddle wheel, such constructions have not been entirely satisfactory for accurately and linearly measuring flow rates in the range of 20 to 40 milliliters per minute, as is required in certain critical applications such as kidney dialysis machines. Additionally, such prior art constructions typically employ paddle wheel constructions which introduce interruptions in the fluid flow as the paddle rotates past the fluid inlets and are unbalanced because of the necessity of metallic substances in one of the paddle wheels for the sensing technique employed.
Thus, in the area of the critical measurement of relatively low fluid flow rates, such as in the 20 to 40 milliliter range, there has been a need for a substantially linear and accurate sensor for measuring fluid flow. The present invention satisfies that need.